Ceramic materials are commonly prepared by mixing powdered ceramic oxides such as magnesia, alumina, titania and zirconia, in a slurry along with additives, such as dispersants and binders. The slurry may be spray dried to produce ceramic particles. The particles are formed into an aggregate structure, called a "green ceramic," having a desired shape and subsequently subjected to a severe heat treatment known as sintering. The sintering process converts the green ceramic into a cohesive "fired ceramic", having a nearly monolithic polycrystalline ceramic phase.
The binder serves to hold the ceramic particles of the green ceramic in the desired shape after forming. The binder can also provide lubrication while the particles are pressed. Preferably, the binder combusts or vaporizes completely during the sintering process leaving no trace of the binder in the fired ceramic. In performing these functions, binders significantly affect the properties of the fired ceramics which are ultimately produced.
In commercial practice, poly(vinyl alcohols) are widely used as ceramic binders. Additionally, poly(ethylene oxide) and ethylene-vinyl acetate copolymers reportedly have been used as binders for particulate material, such as granular silica gel.
For example, polymeric binders containing substantially hydrolyzed copolymers made from monomers having ester or amide functional groups, poly(vinylformamide) or a copolymer of vinyl alcohol and vinyl amine are disclosed in U.S. Pat. Nos. 5,358,911; 5,487,855 and 5,525,665.
Spray drying is an evaporative process in which liquid is removed from a slurry containing a liquid and a substantially non-volatile solid. The liquid is vaporized by direct contact with a drying medium, usually air, in an extremely short retention time, on the order of about 3 to about 30 seconds. The primary controlling factors in a spray drying process are particle size, particle size distribution, particle shape, slurry density, slurry viscosity, temperature, residence time, and product moisture.
The viscosity of the slurry must be suitable for handling and spray-drying. Although spray-drying equipment conditions may be adjusted to handle a variety of viscosities, larger particles will usually result from higher viscosity slurries.
Those of ordinary skill in the art are familiar with the spray-drying process used in the production of ceramic materials, and will be able to optimize the control factors of spray-drying to best advantage. Alternatively, the spray drying or dry pressing processes may be replaced by other well known forming methods, such as granulation, tape casting and slip casting.
Spray drying of the slurry produces substantially dry, free-flowing powder particles which contain the ceramic, the binder and the optional materials described above. The dry particles are granules which are generally spheroidal in shape and have an effective diameter of about 50 to about 300 micrometers. Typically, about 0.5 percent to about 8 percent of the binder, based on the dry weight of the ceramic powder, is present in the dry particles.
In granulation, a mixture of dry powder or powders is mixed or rolled, commonly in a barrel shaped apparatus. Water and/or a binder solution is sprayed into the mixing powder causing aggregation of the small particles into larger granules. The size of the granules is controlled by the amount of material sprayed into the powders and the speed with which it is sprayed. Granulated powders may be screened to a desired size and pressed to shape in a pressing operation prior to sintering. Alternatively, the granules themselves may be the desired product and may be sintered directly.
Tape casting is commonly used to produce thin substrates for the electronics industry. In the process, a thick ceramic slurry containing ceramic powder, dispersant and binders is prepared. This slurry is cast onto a smooth surface such as a MYLAR or plastic sheet and the thickness is controlled by passing the sheet under a blade which smoothes the slurry surface and scrapes off excess material. The slurry tape is dried to a plastic state and cut and shaped to specification. The amount of binders present in tape casting is very high, typically on the order of 15 to 20 wt. % of the ceramic powder mass.
In fluidized bed spray drying, small "seed" particles are placed in a column and hot air is blown into the seed powder from below suspending the particles in the column. A ceramic slurry is sprayed onto the seed particles from above, causing them to grow. When the particles reach a large enough size, they are siphoned out of the dryer while more seed particles are introduced. This process can produce powder for further forming processes, or the powder itself may represent the desired product, in which case it would be sintered to produce the final ceramic.
The dry particles are compacted to produce an aggregate, green ceramic structure. Preferably, the particles are compacted by pressing in dies having an internal volume which approximates the shape desired for the final fired ceramic product. Alternatively, the particles are compacted by roll compacting or other well-known compacting methods. The spray dried blend of powder, binder, and optional surfactants and lubricants is relatively free flowing so that it can enter and closely conform to the shape of the pressing dies.
Inside the dies, the dry particles are subjected to a pressure which is typically in the range of about 2000 to about 50,000 psi. Pressing the particles produces an aggregate structure, called a green ceramic, which retains its shape after removal from the die.
One forming technique used for spray dried or granulated material is roll compaction, also referred to as roll pressing. This technique takes a dry powder and crushes it between two rollers in a continuous process. This process produces sheets of ceramic of various widths and thicknesses. These sheets can be cut to shape and sintered to produce the final ceramic body. The process is commonly used to produce ceramic substrates for the electronics industry.
Dry pressing involves filling a shaped die with spray dried or granulated powder and pressing it at high pressures. The pressing occurs through movable pistons at the top and/or bottom of the die cavity. The process can be used to produce fairly complex geometries in a single forming step. The ceramic body that results is ejected from the die and sintered to produce a final ceramic product.
Isostatic pressing is similar to dry pressing in that a ceramic powder is pressed in a die cavity. In isostatic pressing, however, all or part of the die wall consists of a flexible material. After filling the die cavity with powder, the die is submerged in a liquid pressure chamber and pressure is applied to squeeze the die and compact the powder. Unlike dry pressing, no movable parts are involved. Isostatic pressing is commonly used on large or very long parts to minimize cracking or lamination of the final ceramic green body.
Extrusion involves the pushing of a concentrated, plastic, slurry through an orifice. The orifice is of the size and shape of the desired ceramic body. This process is commonly used to produce ceramic tubes or similarly shaped pieces. The slurry used is prepared from dry powders which are mixed with water, organic binders and lubricants, and a coagulant. This slurry is usually predried in a filter press or similar apparatus to remove excess water and thicken the slurry to a plastic material. The material is then extruded through a press which is either piston or screw driven. The extruded material is cut to length, dried, and sintered.
Jiggering is commonly used in the whiteware industry to shape an extruded or filter pressed ceramic slurry. Typically, a portion of the plastic slurry is placed on a rotating wheel and shaped by rollers and/or knife blades to a desired geometry. This body is then dried and sintered.
Another ceramic forming method, that is used for parts of complex shape, is slip casting. In slip casting, a concentrated ceramic slurry (slip) is poured into a mold with an internal shape of the desired ceramic body. The slurry used must be highly concentrated to prevent settling of particles and/or excessive shrinkage during drying. At the same time, the slip must be fluid enough to completely fill the mold and allow escape of air bubbles. The presence of a polymeric binder adds strength to the cast body preventing breakage during mold removal and handling of the body prior to sintering.
Heating the aggregate structure drives off volatile materials such as water, and burns off organic materials, such as binders, plasticizers, dispersants or surfactants. When a sufficiently high temperature is reached, the particles of the aggregate structure begin to fuse, but do not fuse completely, and become fastened to one another to produce a relatively strong fired ceramic material having essentially the desired shape.
The slurry is, for example, spray dried to produce substantially dry particles which include the polymer. The particles are preferably pressed to produce an aggregate, green ceramic structure and heated to produce a fired ceramic material. Alternatively, the particles can be formed into an aggregate, green ceramic structure by roll compaction or other well-known methods.
Many references describe the use of polymers as aids in the manufacture of ceramics. However, such polymers are generally formed by radical-induced polymerization of vinylic monomers. Substantially hydrolyzed copolymers formed from vinylic esters and amides are disclosed in U.S. Pat. Nos. 5,358,911; 5,525,665 and 5,487,855 for ceramics production. A hydrolyzed terpolymer formed from maleic anhydride, N-vinyl pyrrolidinone and a third vinyl monomer for preparing ceramic oxide materials is disclosed in U.S. Pat. No. 5,266,243. A method for dispersing one or more ceramic materials in an aqueous medium utilizing a polymer formed from hydroxy functional monomers and acid-containing monomers is disclosed in U.S. Pat. Nos. 5,532,307 and 5,567,353. A method for preparing sintered shapes utilizing the reaction product of an amine other than an alkanolamine with a hydrocarbyl-substituted carboxylic acylating agent is described in U.S. Pat. No. 5,268,233, and a method for increasing green fracture strength of a ceramic part utilizing polymers of vinylic monomers is disclosed in U.S. Pat. No. 4,968,460. Nylon-type resinous products have also been utilized for improving wet strength in papermaking in U.S. Pat. No. 3,250,664.
Polyamides, and their cross-linked derivatives, are known. Water soluble polyamide syntheses are disclosed in U.S. Pat. Nos. 5,053,484 and 5,324,812. Moreover, condensation polymers have been cross-linked and utilized as Yankee Dryer Adhesives as disclosed in U.S. Pat. No.5,382,323. Cross-linked polyamino-polyamides have been disclosed as cosmetic compositions for hair in U.S. Pat. No. 4,277,581. Amino-polyamide-acrylamide adducts reacted with polyaldehydes have been disclosed as wet strength agents in U.S. Pat. No.3,607,622. Polyamides are reacted with epichlorohydrins to form resins for use as wet strength agents in U.S. Pat. Nos. 2,926,154 and 2,926,116.
Furthermore, it is known that some post-polymerization modifications may enhance the activity of polymers utilized in ceramics manufacturing. For example, U.S. Pat. No. 5,268,233 describes post-treatments with reagents such as aldehydes and epoxides of polymers which are the result of reaction of alkanolamine and a hydrocarbyl-substituted carboxylic acid acylating agent. Use of cross-linking agents such as epoxides and polyhydric alcohols to increase green fracture strength of polymers which are the result of free radical polymerizations has been disclosed in U.S. Pat. No. 4,968,460.
However, none of these references disclose the cross-linked poly(aminoamide) condensation polymers described herein, for ceramics manufacturing.
Therefore, although commercially available binders are satisfactory for many applications, a need still exists for improved binders which provide still greater strength and/or green density in green ceramic materials. Greater green strength is advantageous because it reduces breakage during handling of the green ceramics and, generally, is associated with higher quality fired ceramics. Our polymeric binders provide the requisite increase in green strength.